What recorder did you buy? Some are more sensitive than others with the lowest notes. So-called “playing in” is much over-rated — more for the player, in most circumstances. So, have at it!
The two areas of concern with low notes: air stream, finger placement. The low notes require a much slower and very open (“warm”) air stream — as if you are steaming up a mirror.
The other likely problem is with the fingers. With “all fingers down” it’s very easy to overlook a misplaced finger, i.e. a hole that is not fully closed, particularly in the right hand (index finger, for example). You can check for this by lifting and replacing one finger after another.